This invention relates to radially expandable endoprostheses which are adapted to be implanted in a body lumen. An “endoprosthesis” corresponds to an artificial implantable medical device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. These endoprostheses are commonly referred to as stents. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty or valvuloplasty) with apparent success.
The cylindrical structure of stents is typically composed of a scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or planar films of material rolled into a cylindrical shape. In addition, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier. The polymeric carrier can include an active agent or drug. Furthermore, the pattern that makes up the stent allows the stent to be radially expandable and longitudinally flexible. Longitudinal flexibility facilitates delivery of the stent and radial rigidity is needed to hold open a body lumen. The pattern should be designed to maintain the necessary longitudinal flexibility and radial rigidity of the stent.
A number of techniques have been suggested to fabricate stents from tubes and planar films or sheets. One such technique involves laser cutting or etching a pattern onto a material. Laser cutting may be performed on a planar film of a material which is then rolled into a tube. Alternatively, a desired pattern may be etched directly onto a tube. Fabricating a stent from a tube is preferable due to time and cost considerations. Other techniques involve cutting a desired pattern into a sheet or a tube via chemical etching or electrical discharge machining Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. No. 5,922,005 to Richter and U.S. Pat. No. 5,906,759 to Richter.
It is desirable for a stent to have certain mechanical properties to facilitate delivery and deployment of a stent, especially in the bending portions of the stent that are bent during crimping and expansion of the stent. For example, longitudinal flexibility is important for successful delivery of the stent. In addition, radial rigidity and strength are vital characteristics in deployment and for holding open a body lumen. The pattern that makes up the stent allows the stent to be radially expandable and longitudinally flexible. The pattern should be designed to maintain the necessary longitudinal flexibility and radial rigidity of the stent. One technique for strengthening the bending portions of a stent is to laser cut the stent such as to widen the bending portions of the stent. However, upon crimping a stent that includes wider bending portions, oftentimes the stent flips upwards or “chip” when the strut is bent during crimping and/or expansion.
What is needed in the art is a method of fabricating a stent to mechanically strengthen the stent in selected portions.